The electrophotographic process is a widely used printing technology for generating hardcopy documents from electronic data. At the heart of the electrophotographic imaging process is an organic photoconductive (OPC) drum. This drum typically includes an extruded aluminum cylinder coated with a non-toxic organic photoconductive material. A light source, such as a laser or a light emitting diode, is used to create a latent image by selectively discharging portions of a uniform charge field conditioned on the photoconductive material of the drum. There are six generalized stages to the electrophotography process: cleaning, conditioning, writing, developing, transferring and fixing. More details of the basic electrophotographic process will be described below.
The cleaning stage prepares the OPC drum to receive a new latent image by applying a physical and electrical cleaning process. The physical cleaning of the OPC is typically accomplished by a drum-cleaning blade (or wiper blade) and a recovery blade. The wiper blade scrapes any excess toner from the drum and the recovery blade catches the toner and sweeps it into a waste hopper. In the electrical aspect of cleaning, the previous charge field on the drum must be cleared before a new one may be applied. The electrical cleaning of the OPC drum is performed by erasure lamps (usually corona wire technology) or a primary charge roller (PCR), which eliminate the previous charge field and latent image from the drum.
After the drum has been cleaned, it must be conditioned or charged to accept the next image from the light source. A primary corotron (corona wire or PCR) applies a uniform negative charge (usually in the range of −600V to −720 V DC) to the surface of the drum.
Following the conditioning stage is the writing stage. According to this stage, the light source, e.g., a laser beam, is used to selectively discharge portions of the conditioned charge field from the drum surface. This selective discharge of the conditioned charge field creates a latent image on the drum. This is achieved as follows. The metal base of the OPC drum is connected to an electrical ground. The photoconductive material on the OPC becomes electrically conductive when exposed to light. Therefore, the negative charges deposited onto the surface of the drum conduct to the metal base and thence to ground when the photoconductive material is exposed to light, thereby creating the latent image. The latent image area will attract toner in a later stage.
The fourth stage is developing. At this stage, the latent image becomes a visible image. This stage generally requires four major components: toner, a developer roller assembly, a metering blade, and an AC/DC charge. Toner is attracted to the developer roller either by an internal magnet or by an electrostatic charge. The roller carries the toner particles to a metering or doctor blade, where toner tumbles and creates a tribo-electric charge (friction) on the surface of the toner particles. The metering blade then provides for an evenly distributed amount of toner to pass to the OPC drum. Once the toner particle has passed beyond the doctor blade, it is ready to be presented to the OPC drum. The developer roller is then charged with an AC/DC charge from the High Voltage Power Supply. This charge allows the toner particles to “jump” from the developer roller and travel to the OPC drum where it is attracted to the latent image.
At this point, the toner image on the drum is transferred onto a sheet of paper. As the paper is passed under the OPC drum, it is passing over a transfer corotron assembly. The transfer corotron assembly places a positive charge on the back of the page, thus attracting the toner from the drum.
The sixth and final stage is fixing. Also known as fusing, this is the stage in which toner is permanently affixed to the paper. The fuser assembly typically includes a heated roller, a pressure roller, a heating element, a thermistor, a thermal fuse, and, sometimes, a cleaning pad. The heating element is typically placed inside the heated roller, which is usually constructed of aluminum with a Teflon coating. The roller is heated to approximately 355° F. (180° C.). The second roller is usually a rigid foamed silicon rubber. This second roller applies pressure to the heated roller. The paper passes between the two rollers and the heated roller melts the toner particles while the pressure roller presses the toner into the fiber weave of the paper.